1. Field of the Invention
The present invention relates to a printed circuit board including a transmission line that transmits a digital signal.
2. Description of the Related Art
Because of there being a demand for high speed and high definition, the recent digital complex machines and digital cameras are required to transmit signals having large-volume digital signals at high speed. In order to transmit such large-volume data at high speed, it is necessary to increase the number of transmission lines or the transmission speed. Printed circuit boards with their sizes reduced and their densities increased have a limitation in increasing the number of transmission lines. Furthermore, where a cable is used for transmission, an increase in the number of cable cores leads directly to an increase in cost. Furthermore, as the transmission speed increases, significant signal timing differences occur due to skew, resulting in difficulty in providing a setup time and a hold time. Therefore, serial transmission, which enables high-speed transmission of large-volume data with a small number of transmission lines, is widely used.
Serial transmission is a transmission method in which low-speed parallel signals such as a data signal, an address signal and a control signal are serialized in a transmission line to provide a differential output, and the transmitted serial signal is deserialized into parallel signals on the reception side. In this serial transmission method, a serialized data string with a clock signal embedded therein is transmitted and clocks and data are reproduced on the reception side.
Meanwhile, where a high-speed signal is transmitted in a long, lossy transmission line such as a cable, a part of components of the signal may be radiated with the cable as an antenna, affecting the operation of other apparatuses. Therefore, it is necessary to suppress EMI (electromagnetic interference) from the relevant apparatus.
In the clock-embedded serial transmission, data and synchronizing clocks are serialized together, data coded so as to exhibit a logical transition ratio of approximately 50% for each of a high level and a low level is transmitted. Thus, in the transmitted serial data, there are no consecutive multiple bits remaining at a low level or a high level, and a repetitive waveform having a basic cycle of one bit dominantly appears. Accordingly, strong EMI from a serial transmission system is observed in domains of frequencies that are integral multiples of the one-bit cycle of the serial data. Also, it is known that data transmitted by means of a rectangular wave has a spectrum expressed by a sin c function, and has no spectrum in domains of frequencies that are integral multiples of a one-bit cycle. In other words, EMI occurs in the domains of frequencies where there is no spectrum of the transmitted signal. For example, where a transmission rate is 500 Mbps, a cycle of one bit is 0.5 GHz (basic frequency), there is no spectrum of the transmitted signal in domains of frequencies that are integral multiples of the one-bit cycle, and strong EMI occurs in the domains. Where this clock-embedded serial transmission method is employed, EMI occurs in domains of frequencies where there is no spectrum of a transmitted signal, and thus, a band-elimination filter or a notch filter can be used for the differential transmission line. This is because such a band-elimination filter or notch filter filters out a band in the vicinity of frequencies where there is no spectrum of the transmitted signal, enabling actively EMI to be removed without imposing effects on the transmitted signal.
Therefore, it is conventionally known that a band-elimination filter has a configuration in which a serial circuit of a coil and a capacitor is connected in parallel to a transmission line or a configuration in which a parallel circuit of a coil and a capacitor is connected in series to a transmission line. However, in such a band-elimination filter, each circuit element includes a lumped element such as a chip part, and thus, in a high-frequency domain such as a GHz band, a constant value of the part has a very small value. Accordingly, it is difficult to provide desired frequency filter-out by means of its standard parts. Furthermore, it is also difficult to provide desired frequency filter-out because of variation in element values of the parts.
As a solution to the above problem, in order to provide desired characteristics in high-frequency domains and solve the problem of variation in element values of the parts, the technique of forming a band-elimination filter using a distributed element circuit is known. For a high-frequency band such as a GHz band, it is common to form a distributed element circuit using a stripe line formed on a substrate as a distributed element. Where a distributed element circuit is formed using a distributed element, values corresponding to element values of a lumped element are determined by physical dimensions such as a width of the stripe line and the length of the stripe line, and desired frequencies to be filtered out can be controlled by the dimensions to be provided, enabling desired characteristics to be obtained easily.
Japanese Patent Application Laid-Open No. H09-232804 discloses a band-elimination filter including a distributed element, in which a line related to an electrical length of a frequency to be removed is arranged close to a principal transmission line to remove undesired frequency components. This configuration is illustrated in FIG. 12A. In this configuration, a line 105 with one end connected to a ground is arranged close to a principal transmission line 103.
As a passage characteristic of the transmission line configured as described above, the line with the end thereof connected to the ground and having an electrical length corresponding to λ/4 (λ: wavelength) of the basic frequency, attenuates the basic frequency and frequencies that are odd multiples of the basic frequency.
However, the configuration illustrated in FIG. 12A is able to remove EMI in domains of frequencies that are odd multiples of the basic frequency, but cannot attenuate frequencies that are even multiples of the basic frequency, and thus has a problem of providing no EMI removal effect for the frequencies that are even multiples of the basic frequency. FIG. 12B indicates a transmission line attenuation characteristic of the conventional configuration illustrated in FIG. 12A. This characteristic is a band rejection characteristic with 0.5 GHz as a basic frequency, and exhibits attenuation for 1.5 GHz and 2.5 GHz, which are odd multiples of the basic frequency, but exhibits no attenuation for 1 GHz and 2 GHz, which are even multiples of the basic frequency. Accordingly, in the clock-embedded serial transmission, strong EMI occurs in the domains of frequencies that are integral multiples of a basic frequency corresponding to a data transmission rate, and components having frequencies that are even multiples of the basic frequency cannot be removed.